The present invention relates to a method for detecting a focus condition of an imaging optical system comprising dividing a light flux emanating from the imaging optical system into a first light flux which is transmitted through a first region of the imaging optical system and a second light flux which is transmitted through a second region of the imaging optical system, said second region including a portion different from the first region, projecting separately the first and second light fluxes into first and second light receiving element arrays to produce first and second sets of photoelectrically converted output signals, and deriving a correlation between the first and second sets of output signals to detect a focus condition of the imaging optical system due to a lateral shift of first and second images formed on said first and second light receiving element arrays by said first and second light fluxes.
Such a focus detection method has been utilized in various optical machineries such as still camera, television camera and cine camera.
It has been proposed in, for instance, Japanese Patent Application Laid-open Publications Nos. 60,645/73 and 95,221/77 to detect the focus conditions, i.e. forwardly and backwardly de-focused and in-focused conditions of an objective lens by dividing a light flux emanating from the objective lens into two light fluxes transmitted through right and left halves of the objective lens, respectively, by means of a light flux dividing means and by comparing a phase shift of images formed by said divided light fluxes upon a pair of light receiving element arrays. In the former method disclosed in the above mentioned Laid-open Publication No. 60,654/73, the light flux dividing optical system is formed by an oscillating slit which is mechanically moved. In the latter method, a relay lens is arranged behind a predetermined focal plane of the objective lens and thus a relatively large space is required. In both methods, it is difficult to derive with a high sensitivity a correlation of distributions of the photoelectrically converted output signals from the light receiving element arrays which receive selectively the light fluxes transmitted through the right and left halves of the objective lens, respectively. In Japanese Patent Application Laid-open Publication No. 159,259/79 there is disclosed a focus condition detecting method in which an array of micro lenses is used as the light flux dividing optical system and each light receiving element of the arrays is arranged to receive light fluxes emanating from respective micro lenses. The light amount distributions of the light fluxes transmitted through the right and left halves of the objective lens are compared with each other in a physical manner for each light receiving element pair and when the coincidence is detected, the objective lens is considered to be in-focus.
In the last mentioned method disclosed in Japanese Patent Application Laid-open Publication No. 159,259/79, in order to make the two physical amounts coincident with each other, it is necessary to provide the micro lens system, and in the in-focus condition the image must be formed on the micro lens system. However, it is quite difficult to manufacture the micro lens array with a high precision. Moreover, in order to avoid the superimposition of the light fluxes upon each other, it is necessary to pitch the micro lens array to a large extent or to provide small light receiving elements. Thus, if an image has a sharp edge, the edge might situate at an intermediate position between successive light receiving element pairs. In such a situation, the correlation of the images cannot be derived precisely.